The present invention is directed to thermostable back-surface mirrors with controllable reflectance, comprising a substantially transparent glass substrate which is coated with a thermostable mirror coating. The coated glass mirrors can be tempered and bent to a desired shape while remaining reflective. The present invention also is directed to methods for the production of such thermostable back-surface mirrors.
Reflective coatings for transparent glass substrates, such as automotive, architectural or decorative glazing, industrial mirrors, components of scientific or navigational instruments, and optical lenses, are known to those skilled in the art. It would be desirable to have a back-surface mirror which adequately retains its reflective properties under thermal stresses encountered during tempering or bending the glass substrate; however, known mirror coatings are functionally diminished in reflectance or are otherwise degraded to an unacceptable degree when subjected to such high temperatures.
According to prior known methods, the glass substrate is first tempered and shaped prior to coating. Thus, the resulting shaped articles must be coated individually, rather than in bulk as part of a single, large glass sheet. This has the substantial disadvantage of increasing production time, cost and complexity. Also, in order to coat an article which is not flat, e.g, a curvo-planar article, specialized coating apparatus is required, which results in further expense. A reflective glass coating is disclosed by Chesworth et al in European Patent Application 0 301 755. This prior art coating comprises a layer of at least one metal having an atomic number of 22 to 29 and a thin layer of aluminum applied over the metal layer, the latter for thermal protection during tempering or bending. Heat processable, metallic coatings, and vacuum coatings, are disclosed in U.S. Pat. No. 5,705,278 to Gillery et al.
Also known are front-surface mirrors. Front-surface mirrors suffer the disadvantage that it is difficult to achieve adequate long-term adhesion between the substrate and the metal mirror coating. The front-surface mirror coating is applied in a cold coating process as the last step after the glass has been formed and bent to shape. Also known are transparent mirrors, that is, half-mirrors, such as those employing alternating layers of TiO2/SiO2. The deposition of a large number of alternating layers for this type of mirror structure is costly and complex. Some known mirrors are provided with a condensation or ice control mechanism employing electrical heating. A typical structure would include a silver metal film applied on a surface of the glass by silkscreen or other technique, to act as an electrical resistance heater. A black paint layer is then applied over the silver metal film. Both the black paint layer and the silver metal film then must be dried simultaneously at about 600xc2x0 C., during which the glass is bent into the desired shape.
It is an object of this invention to provide thermostable, optionally colored, back-surface mirrors comprising a substantially transparent glass substrate, wherein the mirror coating is thermostable at the tempering or bending temperature of the glass substrate.
It is an object of at least certain preferred embodiments of the invention to provide back-surface mirrors with the desired reflectivity values ranging from above 53% to 85%, having color or no color, which are thermostable at the bending temperature of the glass substrate.
It is a further object of the invention to provide methods for the manufacture of the above-described Thermostable, reflectivity-controllable and color-controllable, back-surface mirrors.
Additional objects and advantages of the present invention will be readily understood by those skilled in the art given the benefit of the following disclosure of the invention and detailed description of certain preferred embodiments.
The present invention provides a highly reflective, thermostable, chromium-based back-surface mirror, optionally being optically colored, comprising a transparent glass substrate and a thermostable mirror coating on a surface of the glass substrate. It should be understood that, as used here, a xe2x80x9cback surfacexe2x80x9d mirror is reflective through the glass substrate, that is, when viewed through the glass substrate from the surface opposite that surface which caries the thermostable mirror coating. In accordance with one aspect, the novel thermostable back-surface mirrors have a reflective chromium-based mono-layer. As used here, xe2x80x9cchromium-basedxe2x80x9d means chromium or any suitable Nixe2x80x94Cr alloy and xe2x80x9creflective chromium-based mono-layerxe2x80x9d means a single layer of Cr or Nixe2x80x94Cr which is the only reflective material in the mirror coating. In certain preferred embodiments, these mirrors have been thermally processed, i.e., tempered and/or heated for bending. The reflective chromium-based mono-layer in such mirrors has a thickness exceeding that of chromium layers employed in prior known mirrors. More specifically, the reflective chromium-based mono-layer is sufficiently thick to maintain mirror-quality, back-surface reflectivity after such heat treatment, even without an overlying protective layer. Exemplary chromium-based mirrors disclosed here employ a chromium or Nixe2x80x94Cr layer 140 nm thick. Embodiments of the present invention employing a soda-lime-silica glass substrate with a 140 nm thick chromium-based mono-layer and no protective material overlying the mono-layer, still have coating capacity and mirror-quality back- surface reflectance, e.g., greater than 53% back-surface reflectance, after tempering and bending. The front surface of the chromium-based mono-layer, which is exposed to atmosphere during the heating, oxidizes to form a self-protective CrOx surface imparting a substantially non-reflective, blue-colored appearance. In contrast, mirrors of the above-mentioned EP application 0 301 755 employ a chromium reflective layer only 60 nm thick and require a protective aluminum layer overlying the front surface to maintain back-surface reflectivity after heat treatment.
In accordance with another aspect of the invention, thermostable back-surface mirrors have a reflective chromium-based layer, i.e., Cr or Nixe2x80x94Cr, together with a reflectivity enhancing metal layer of silver or copper between the glass substrate and the chromium-based layer. In certain preferred embodiments, the reflectivity enhancing metal layer is sandwiched between chromium-based layers. The reflectivity enhancing layer, especially in the case of silver, should be protected against oxidation, both from the glass side and from the outside. The overlying chromium-based layer protects the outside. A buffer layer is used to protect the glass side. The second (i.e., glass side) chromium-based layer serves as such required buffer in the preferred embodiments mentioned above, wherein the reflectivity enhancing silver or copper metal layer is sandwiched between chromium-based layers. Alternative buffer layers between the reflectivity enhancing layer and the glass include a layer of Si. The buffer should be sufficiently thin to be substantially transparent to visible light, so as to maintain back-surface reflectance. It should be sufficiently thick to provide adequate protection against oxidation of the reflectivity enhancing metal layer. A buffer layer also can be used in preferred embodiments of the invention employing a chromium-based mono-layer for reflectivity.
In accordance with another aspect of the invention, the thermostable mirror coating includes an oxide layer between the above-mentioned layers and the glass substrate. In certain preferred embodiments, the oxide layer is a color-forming layer of copper oxide (CuOx) or tin oxide (SnO2). In other preferred embodiments, the oxide layer is a colorless oxide, e.g., WO3. The thermostable mirror coating is reflective through the glass substrate, that is, the mirror is reflective when viewed from the glass substrate surface opposite that which carries the thermostable mirror coating. Preferred embodiments of the back-surface thermostable mirrors disclosed here, having the most superior back-surface reflection, comprise a silver or a copper layer sandwiched between a thin Cr or Nixe2x80x94Cr layer and a thick protective Cr or Nixe2x80x94Cr layer. The thin chromium layer (as defined above, meaning a Cr or Nixe2x80x94Cr alloy) is between the silver or copper metal layer and the glass substrate. The thicker chromium layer overlays the silver or copper layer. The thicker chromium layer preferably is substantially thicker than the thin chromium layer, preferably being at least about 17 times as thick, for example, 35 to 100 times as thick. The oxide color-forming layer or transparent oxide is positioned between the glass substrate and the thin chromium layer. The oxide layer is sufficiently thick to provide any desired degree of color, but is not so thick as to unacceptably diminish the reflectivity of the thermostable mirror coating. In accordance with certain preferred embodiments, coloration can be provided through the use of tinted or so-called body-colored glass substrate.
The invention provides a significant advance over prior known mirrors. It provides thermostable back-surface mirrors which are optionally colored. Moreover, the thermostable back-surface mirrors of this invention can be manufactured with excellent control over the degree of coloration, and also with excellent control over the degree of reflectivity. Mirrors of this invention can be highly reflective, like traditional (non-thermostable) silver mirrors, or less reflective. Reflectance can be controlled by adjusting the thickness of the silver or copper metal reflectivity enhancing layer and by other suitable coating adjustments. The degree of color can be controlled by choice of a colorless oxide layer, as mentioned above, or a copper oxide or other color-imparting oxide layer at a thickness corresponding to the desired degree of color. Color also can be controlled by selection of a colored or uncolored glass substrate. As used here, reference to the substantially transparent glass substrate being colored are calculated using the Commission Internationale de L""Eclairage (CIE) color difference equation:
E=[(L*)2+(a*)2+(b*)2]xc2xd
where a*, b* and L* are color coordinates in CIE uniform color space.
It is highly advantageous that the back-surface mirrors disclosed here are thermostable, i.e., able to withstand the thermal stresses of tempering and/or bending the glass substrate. Thus, considerable reduction in cost and complexity can be achieved in the manufacture of, for example, automotive mirrors and industrial mirrors by providing the thermostable mirror coating disclosed here onto a large flat glass substrate using common architectural glass coaters. Subsequently, the glass substrate with the thermostable mirror coating can be cut to size and then bent and/or tempered. The thermostable mirror coating on the individual cut mirrors, after such bending and tempering, retains its reflectivity to a degree acceptable for the above-mentioned uses, including, e.g., industrial security (or xe2x80x9cone-wayxe2x80x9d) mirrors for supermarkets and the like, and for motor vehicle mirrors, etc. An automotive application of the thermostable mirror is as a rear-view car mirror eliminating the blind spot. Further, the invention provides reflective-coated substrates having enhanced decorative properties due to coloration, by virtue of which the claimed articles of manufacture are suitable for use in luxury applications, e.g., tinted mirrors, interior or exterior architectural design and fashion eyewear or other accessories.
In accordance with one aspect, a temperable and bendable back-surface mirror comprises an optically transparent glass substrate and a thermostable mirror coating on a surface of the glass substrate. The thermostable mirror coating is formed of a reflective silver or copper film protected by chromium layers formed of chromium or nickel-chromium alloy, optionally along with an oxide layer between the first chromium layer and the surface of the glass substrate.
In accordance with another aspect of the invention, a tempered back-surface mirror is provided, comprising an optically transparent glass substrate carrying a thermostable mirror coating as disclosed and described above. The glass substrate has been subjected to a tempering step in which a temperable and bendable back-surface mirror, as described above, is subjected to temperatures sufficiently high to soften the glass substrate. After heating, the mirror is rapidly cooled so as to temper or toughen the glass. Optionally, the mirror is also bent during such heating step.
In accordance with another aspect of the invention, a process is provided for making tempered back-surface mirrors. Specifically, the inventive process disclosed here comprises a combination of steps, including: providing a substantially transparent glass substrate (clear or colored); forming a thermostable mirror coating on a surface of the glass substrate in accordance with the above disclosure, most preferably by first depositing a substantially transparent oxide layer (e.g., a color-forming oxide layer), followed by a thin chromium layer formed of chromium or nickel-chromium alloy over the oxide layer and then a reflectivity-enhancing metal layer of copper or silver, and then a thick protective chromium or nickel-chromium layer deposited on top of the reflectivity enhancing layer; and then tempering the back-surface mirror by heating it to the tempering temperature of the glass substrate and subsequently cooling. The thermostable mirror coating remains reflective after such tempering step, and in preferred embodiments remains reflective to a degree acceptable for such intended purposes as automotive rear-view mirrors, etc. In fact, it is a significant advantage and a novel feature of certain preferred embodiments, that the reflectivity of the back-surface mirror may actually increase in the tempering or bending step, such that the reflectivity of the resulting, i.e., thermally treated, mirror is higher than that of the mirror before it was thermally treated. Typically, back-surface mirrors formed in accordance with the present invention have post-tempering reflectance (again, measured from the glass side) of from about fifty-four to about eighty-three percent (54%-83%).
The reflectance achieved by heat treated (i.e., tempered or bent) mirrors of this invention is about 54% for certain embodiments in which the reflectivity enhancing silver or copper layer has a thickness approaching zero. A sufficiently thick silver layer results typically in about 83% reflectance (again, meaning back-surface reflectance). Advantageously, any reflectance between these two values can be achieved by selecting a suitable intermediate thickness for the reflectivity enhancing layer of silver or copper. It will be within the ability of those skilled in the art, given the benefit of this disclosure, to select suitable film thicknesses to achieve a desired degree of reflectance.
The above values are for mirrors of this invention having optically opaque chromium reflective layers. Correspondingly lower reflectance values are achievable in accordance with alternative embodiments of this invention intended for use as one-way mirrors. One- way mirrors or half-mirrors of the invention employ reflective chromium layers and a reflectivity enhancing silver or copper layer (along with a buffer and/or oxide layer as needed) which are sufficiently thin as to be partially transparent to visible light. It will be within the ability of those skilled in this area of technology, given the benefit of this disclosure, to select sufficiently thin chromium layers and a sufficiently thin layer of silver or copper to achieve a thermostable, back-surface, one-way mirror. Similarly, back-surface mirrors with a color-forming layer in accordance with the present invention, having a thermostable mirror coating on a surface of a glass substrate, may be filly reflective or partially-reflective. Examples of partially-reflective mirrors in accordance with the present disclosure include xe2x80x9cone-wayxe2x80x9d glass and mirrored optical glass, which must reflect light without preventing viewing through the mirror by an observer positioned behind the reflective coating (relative to the light source). Back-surface mirrors in accordance with the present invention which are intended for use as a half-mirror or partially-reflective mirror, with or without a color-forming layer, typically will have reflectance of visible light of at least fifteen percent (15%), more preferably twenty to forty percent (20-40%). Correspondingly, transmittance of visible light for such half-mirrors in accordance with the present invention, will be at least about five percent (5%), more preferably ten to forty percent (10-40%). Visible light absorption is about 50% for such one-way or half-mirrors having 5% transmittance, and about 40% for such mirrors having 40% transmittance.
The thermostable mirror coatings employed in the back-surface mirrors of the present invention are xe2x80x9cthermostablexe2x80x9d in the sense that they are stable at the tempering and/or bending temperature of the glass substrate. More specifically, the thermostable, reflective mirror coatings have the reflectance properties described above after being subjected to the high temperature heating cycle necessary for such bending or tempering of the glass substrate. In addition, the term xe2x80x9cthermostablexe2x80x9d means that the back-surface mirrors disclosed here retain their mechanical properties, such as body integrity, surface continuity, tensile strength and adhesiveness (e.g., at the interface of coating and substrate). In accordance with certain preferred embodiments of the invention, the reflective thermostable mirror coatings are thermostable as here described at temperatures of at least 400xc2x0 C., more preferably at least between about 550xc2x0 C. and 650xc2x0 C.
As used herein, the term xe2x80x9cbendablexe2x80x9d refers to the back-surface mirror being bendable at elevated temperatures to assume and maintain a non-planar body shape. In accordance with certain preferred embodiments, the back-surface mirror is bendable from a planar, that is, flat form into a curvo-planar body shape having a curve or angle of at least 1 degree without breakage or other physical damage, such as crazing, and without substantial loss or diminution of transparency to visible light, strength, etc., at a temperature of at least 600xc2x0 C.
Preferably, the reflective thermostable mirror coating lies directly on the surface of the substrate. As used here and in the appended claims, the reflective, thermostable coating is said to be xe2x80x9cdirectly onxe2x80x9d or to xe2x80x9cdirectly overliexe2x80x9d the glass substrate if no other material or coating is positioned between them. In this regard,. the coating may be said to lie directly on the substrate notwithstanding that there may be a slight transition zone between them, e.g., involving migration of the material of the coating into the substrate and/or interface reaction products. In certain embodiments, especially where no oxide layer is employed between the glass substrate and the first chromium layer, it is presently understood that an oxide interface, believed to be chromium oxide, forms between the glass and the chromium layer during the heat treatment. Consequently, reflection is enhanced after heat treatment in certain preferred embodiments, particularly at temperatures above about 400xc2x0 C., because of the anti-reflection (xe2x80x9cARxe2x80x9d) and transparency enhancing properties of such chromium oxide. Adhesion also is improved. This is presently understood to involve the attraction of oxygen from the glass. In addition, the chromium layer (of Cr or Nixe2x80x94Cr) will have a protective oxide layer, even at elevated temperatures. The thickness of such a protective oxide layer is believed to be about 40 nm at 650xc2x0 C. For this reason, the (thicker) chromium layer overlying the reflectivity enhancing layer provides excellent long-term protection, rendering the back-surface mirrors disclosed here advantageously shelf-stable even without an auxiliary protective layer, e.g., without the aluminum protective layer used in the above-mentioned European patent application No. 0 301 755. One-way or half-mirror embodiments of the invention disclosed here, having a top-most (or outermost) chromium layer which is less thick, preferably further comprise a top-most film of oxide, preferably stoichiometric oxide, for enhanced protection at elevated temperatures against volume changes and resultant micro cracks in the mirror coating.
In accordance with certain preferred embodiments of the invention, an oxide layer is employed with the chromium layers and reflectivity enhancing layer. As disclosed above, the oxide layer can be a color-forming CuOx layer located at the interface of the mirror coating with the glass substrate. In addition, in certain embodiments, an oxide coating of CuOx or SnO2 or the like can be used at both surfaces of a glass substrate for a double-sided, colored mirror. In addition, in certain half-mirror embodiments employing thinner chromium layers, an oxide layer can be employed as a protective overlayer, i.e., as an outermost layer to protect the other layers of the mirror coating from environmental attack. Such half-mirror embodiments preferably are otherwise as disclosed above.
In accordance with certain preferred embodiments, the back-surface mirrors disclosed here comprise a tinted or body-colored glass substrate. Thus, the glass substrate onto which the reflective thermostable mirror coating is deposited will have color properties additional to any contributed by a copper oxide (slightly absorbing) or any other transparent oxide, e.g., SnO2, layer of the thermostable mirror coating. Numerous different tinted glasses suitable for use in the present invention are commercially available and will be readily apparent to those skilled in the art given the benefit of the present disclosure. In certain preferred embodiments, the substantially transparent glass substrate is formed of soda-lime-silica glass, borosilicate glass, aluminosilicate glass, vycor, fused silica or vitreous glass or highly transparent low-iron glass.
In accordance with certain preferred embodiments, the reflective thermostable mirror coating is deposited by sputtering in one or a series of sputter stations arranged sequentially in a single sputtering chamber through which the transparent glass substrate passes at constant travel speed. Suitable partitions, such as curtains or the like, separate one sputter station from the next within the sputtering chamber, such that different deposition atmospheres can be employed at different stations. A reactive atmosphere comprising oxygen, for example, can be used at a first station to deposit an oxide, e.g., the above described color-forming copper oxide or tin oxide layer of the thermostable mirror coating, followed by a non-reactive atmosphere comprising argon, for example, at a second station to deposit the chromium layer of chromium or nickel-chromium. For a thermostable mirror with a reflectivity enhancing film, a silver or copper target can be used at the subsequent station with non-reactive sputtering conditions. The top chromium layer is then deposited under conditions similar to those used for the first chromium (or Nixe2x80x94Cr) layer. In a typical embodiment formed in this way, the resulting film thicknesses (listed in the following order: Glass/Cr/Ag/Cr) are: 50/250/1400 angstroms, respectively. Thus, in such typical embodiment, the chromium layer closer to the glass is formed of Cr about 50 Angstroms thick; the reflectivity enhancing layer is formed of Ag about 250 Angstroms thick; and the outermost chromium layer overlying the silver is formed of Cr about 1,400 Angstroms thick.
It will be apparent to those skilled in the art in view of the present disclosure, that the present invention is a significant technological advance. Preferred embodiments of the back-surface mirror disclosed here, comprising a reflective thermostable mirror coating on the surface of an optically transparent glass substrate, have excellent performance characteristics, including advantageously good color control and reflectance control, and advantageously high physical integrity. Also, temperable and bendable back-surface mirrors comprising such thermostable mirror coating on a glass substrate can be stored for months and even years without substantial degradation. Thus, a quantity of such unformed mirror stock can be maintained in storage for extended periods of time, and then cut to desired size and subjected to a tempering and bending operation when needed. Also, a buffer or color-forming oxide layer and/or other oxide layer(s), the chromium layer formed of chromium or nickel-chromium alloy and any reflectivity enhancing layer can be deposited by commercially known and available sputtering techniques at advantageously high deposition rates, even employing advantageously low deposition power densities. Specifically, regarding the oxide layer, the deposition of tin oxide or copper oxide is economical, as both tin and copper can be sputtered from a pure metal target which is inexpensive and is not greatly subject to target poisoning. The resulting fast production speeds, combined with low materials costs, yields corresponding savings in production costs.
A 3 nm to 50 nm thick copper oxide layer or 10 nm to 100 nm tin oxide layer is sufficient to confer perceptible color to a back-surface mirror or half-mirror as disclosed here. In addition, both copper oxide and tin oxide exhibit very good adhesivity to the glass substrate, and the high density of the copper oxide color layer employed in the bendable, coated articles of the invention, which is found to be as high as bulk value or nearly bulk value, results in excellent durability. This results in advantageously good physical stability and long color retention for the coated articles disclosed here. Also, copper oxide has an advantageously low absorption coefficient in the visible and infra-red regions, together with an advantageously high refractive index similar to silicon, e.g., 3.5 for 550 nm. In short, the copper oxide color layer of the back-surface mirrors disclosed here has advantageous thermal and spectral properties, robust deposition properties and excellent mechanical film properties.
Additional features and advantages of the various embodiments of the present invention will be further understood in view of the following detailed description of certain preferred embodiments.